The present invention relates to a copying machine, a printer, a facsimile machine and a color image forming apparatus of an electrophotographic method having the aforesaid functions, and in particular a color image forming apparatus having an intermediate transfer member on which color toner images are superimposed.
What is commonly known as an image forming apparatus of an electrophotographic method using an intermediate transfer member is one wherein a toner image formed on an image carrier representing a photoconductor is transferred onto the intermediate transfer member, and the toner image on the intermediate transfer member is transferred onto a transfer material (also called a recording sheet or a sheet). In such a color image forming apparatus, the color images formed has various toner layers (adhesion amount) from half tone in a single color to superimposed three solid colors. Generally, a condition of secondary transfer is set to suit a toner adhesion amount of single solid color. However, when an amount of toner adhesion is large because of super imposed two solid colors, a transfer rate of the toner in a bottommost layer is low so that color shading tends to occur.
As a countermeasure for such a phenomenon, it is known that a good transfer performance can be obtained by reducing the electric potential of a superimposed toner layer to that of single solid toner layer. As a means for it, prior to secondary transfer, the electric potential of the toner layer on the intermediate transfer member is discharged through corona discharge. In particular, in order to adjust the electric potential of the toner layer to a predetermined electric potential, a means to discharge using scorotron electrode is effective.
However, in neutralization for the toner layer, if neutralization is excessive, the adhesion force for toner to the intermediate transfer member becomes weak and thereby, images are deteriorated due to toner dispersing. Therefore, an appropriate electric potential of the toner layer is needed.
Each of Patent documents 1 and 2 discloses a technique to charge toner images before secondary transfer, in which, however, voltage of the same polarity as that of toner is applied to the discharge electrode (wire), because its purpose is pre-transfer charge.
(Patent Document 1) Unexamined Japanese Patent Application Publication No. Tokkaihei 10-274892
(Patent Document 2) Unexamined Japanese Patent Application Publication No. Tokkaihei 11-143255
Since the toner layer voltage relates to a charge amount of toner as well as to an amount of toner adhesion, it varies depending on environmental and condition of use. To obtain good transferability of secondary transfer, any toner layer voltage is needed to be adjusted to an appropriate toner layer voltage after neutralization by the discharging device provided before the secondary transfer.
An object of the present invention is to provide an image forming apparatus capable of obtaining an image having neither color shading nor toner dispersing wherein a charge amount of toner is predicted to control an discharging device before the secondary transfer thereby the secondary transfer is carried out in good condition.